The present invention relates to a balancer device of an engine and belongs to a technical filed of measures against engine vibration and noise.
Generally, the balancer device equipped at an engine for a motor vehicle and the like for suppressing vibration and noise thereof has a structure in which a pair of balancer shafts having weight portions are disposed in parallel to a crankshaft so as to be supported rotatably at a case member which is provided at a lower portion of an engine block (an assembly of engine parts, such as, for example, a cylinder block, a lower block, a bearing cap, a bearing frame) supporting a crankshaft rotatably. Also, there are provided an endless chain winding around a driving sprocket attached to one end of the crankshaft (for example, to a front end of the engine) and a driven sprocket attached to one end of either of the balancer shafts (first balancer shaft), and a pair rotational gears provided on the both balancer shafts so as to engage with each other, whereby the balancer shafts are rotated respectively in opposite directions by the crankshaft. The rotation of the balancer shafts with such weight portions generate a vibrating force in a vertical direction, which can cancel a vibrating force in the vertical direction which is caused by the combustion occurring in respective engine cylinders, thereby suppressing the engine vibration and noise.
Japanese Patent Laid-Open Publication No. 2001-74104 (FIGS. 2 and 3) (its corresponding U.S. Pat. No. 6,471,008 and EP Patent Publication No. 1081344) discloses a structure in which a pair of weight, portions are provided on respective balancer shafts in the axial direction, the rotational gears are provided in front of front-side weight portions, and rear-side journal portions formed between the weight portions and front-side journal portions formed in front of the rotational gears are supported respectively at front-and-rear bearing portions at the case member, whereby the balancer shafts are supported rotatably at the case member.
Particularly, in the structure disclosed in the above-described publication, the case member is comprised of an upper case and a lower case which are provided facing each other, each of the rear-side bearings is formed in a hole shape by respective half-split portions of the upper and lower cases, and each of the front-side bearing portions is formed in a hole shape by a hole which has been formed integrally at either one of the upper and lower cases.
Herein, when the balancer shafts are assembled in the case member, before attaching the upper and lower cases to each other, each of one end of the balancer shafts is inserted into the hole of the front-side bearing portions to locate the front-side journal portions at the front-side bearings portions, and then the rear-side journal portions are positioned on the half-split portions of the rear-side bearing portions. Then, the upper and lower cases are attached to each other so as to form the rear-side bearing portions in the hole shape, whereby the rear-side journal portions are supported at the rear-side bearing portions.
Meanwhile, in order to attain surely a suppression effect of engine vibration and noise by the balancer shaft, the weight portions of the balancer shaft are located at a central portion in the axial direction of engine (for example, in the case of an inline 4-cylinder engine, at portions corresponding to a second and third cylinders thereof). On the other hand, in order to provide a proper and sure endless chain winding, the driven sprocket of the first balancer shaft is located so as to align with the driving sprocket of the crankshaft, so that the driven sprocket is provided at the one end of the engine in the axial direction (for example, to the front end of the engine). Accordingly, the first balancer shaft with the driven sprocket is configured so as to extend beyond the second balancer shaft without the driven sprocket at the above-described one-end side thereof, and the driven sprocket is attached to the shaft extension portion. Also, a tension by the endless chain is applied to the driven sprocket. Accordingly, if the first balancer shaft is not supported rotatably near the driven sprocket, the above-described shaft extension portion of the first balancer shaft is apt to bend and thereby causes vibration of the driven sprocket. As a result, there occurs a problem that vibration and noise may be caused by the balancer device itself.